The angular distribution of light coming from a backlight is important in determining both the efficiency and the viewing angle of the display that it illuminates.
Edge lit backlights guide light from the edge of the light guide to points all over the area of the output of the light guide. Light confined within the guide and propagating within the guiding angle of the guide, may be scattered by scattering structures, such as point like defects, on the surface of the guiding layer. Powdered TiO2 is an example of a suitable scattering material. An illustration of such a system is shown in FIG. 1.
In FIG. 1, LEDs (1) are arranged at the edge of a transparent polymer core light guide layer (2). Light (3) from the LEDs propagates through the light guide layer and is scattered through approximately 90° by scattering structures, e.g. point like defects (4). In the figure shown, the device is viewed from above as indicated; the main light output surface is indicated at (5). Typically, the core layer is unclad, relying on the refractive index contrast between the core and surrounding air to provide the guiding effect. Light scattered out of this type of structure is emitted from the top surface over a full hemisphere of output angles.
In an attempt to improve efficiency, it is known to arrange the scattering structures into small dots and position a microlens of a low refractive index material directly above each dot. The combination of the dot and microlens provides some degree of control over the range of output angles provided by the backlight. However, such a system does not always provide as much control over the range of output angles as may be desired for a particular system. Also, the transmission of the overall system may be reduced as the transmission of the liquid crystal screen positioned in front of the backlight typically may be reduced for light incident away from normal incidence.
It is an objective of the present invention to provide at least alternative, and preferably, improved light guide devices possessing, for example, improved optical efficiency. As such, the present invention is partly based on the finding that the range of angles of light which may be usefully collected from an arrangement such as that described above may be greatly extended by incorporating an intermediate low refractive index layer between higher refractive index elements.